The present invention relates to a hydraulic control system for an automatic transmission, and more particularly, to a hydraulic control system for an automatic transmission that enables the advantages of one-way clutches to be optimally used during 1⇄2, 3⇄4, and 4⇄2 shifting.
Conventional automatic transmissions used in vehicles typically include a torque converter, a power train realized through a multi-stage gearshift mechanism that is connected to the torque converter, and a hydraulic control system that selectively operates one of a plurality of operational elements of the power train according to a driving state of the vehicle.
In designing such an automatic transmission, a design concept and plan are formulated based on a variety of factors such as performance, durability, reliability, mass-producibility, and manufacturing costs. After selecting a design concept, development is pursued in three broad areas that include mechanical operation, hydraulic control, and electronic control.
The power train, which falls under the mechanical operation category, is realized through a compound planetary gear set. The compound planetary gear set includes at least two simple planetary gear sets and performs control into a required shift stage. Hydraulic control, which is performed by a hydraulic control system, is used to control the power train. The hydraulic control system includes a pressure regulator for regulating hydraulic pressure generated by operation of an oil pump, a manual/automatic shift controller for forming a shift mode, a hydraulic pressure controller for controlling shift feel and responsiveness to enable smooth shifting, a damper clutch controller for operating a damper clutch of a torque converter, and a hydraulic pressure distributor for supplying suitable hydraulic pressures to friction elements.
The distribution of hydraulic pressure by the hydraulic pressure distributor is varied by solenoid valves that are On/Off controlled and by solenoid valves that are duty controlled, both types of control being performed by a transmission control unit. Accordingly, selective operation of the friction elements is realized to effect shifting into shift ranges and speeds.
In such an automatic transmission, although all the advantages of an automatic transmission over a manual transmission are provided (e.g., ease of driving), the generation of significant shift shock nevertheless remains a problem. To minimize shift shock, it is necessary to smoothly control clutches and brakes of the power train. In this regard, more effective than the most precise electronic control is the mounting of a one-way clutch.
In the case where shifting is performed during an already ongoing shift process, good responsiveness can be expected with the use of a one-way clutch. Because of such advantages, much research is being performed to improve shift feel with the use of two one-way clutches.
FIG. 1 shows a schematic view of a conventional four-speed automatic transmission power train, in which one-way clutches are used.
Rotational force generated by an engine E is transmitted to an input shaft 2 through a torque converter. The input shaft 2 transmits the received torque to first and second single pinion planetary gear sets 4 and 6, and shifting is realized through the complementary operation of the first and second single pinion planetary gear sets 4 and 6. Clutch hook-up, through which output is effected, is realized via a transfer drive gear 10, which is connected to a planet carrier 8 of the first single pinion planetary gear set 4.
In the description below, a sun gear 12, the planet carrier 8, and a ring gear 14 of the first single pinion planetary gear set 4 will be preceded by the word xe2x80x9cfirstxe2x80x9d (e.g., the first sun gear 12); and a sun gear 16, a planet carrier 18, and a ring gear 20 of the second single pinion planetary gear set 6 will be preceded by the word xe2x80x9csecondxe2x80x9d.
In a state where the first planet carrier 8 is fixedly connected to the second ring gear 20, the first sun gear 12 is connected to the input shaft 2 via a first friction element C1. The first friction element C1 operates in all forward speeds. Further, the second planet carrier 18 is connected to the input shaft 2 via a second clutch C2, which operates in forward third and fourth speeds, and the second sun gear 16 is connected to the input shaft 2 via the third clutch C3, which operates in a reverse R range.
Also, the second planet carrier 18 is connected to a transmission housing 22 through a first brake B1 and a first one-way clutch F1, which are mounted in parallel, and is connected to a fourth clutch C4 through a second one-way clutch F2, the fourth clutch C4 being mounted in parallel to the first ring gear 14. Also, the second sun gear 16 is connected to the transmission housing 22 through the second brake B2.
In the power train described above, shifting is realized by operation of the friction elements, which are controlled by the transmission control unit. Referring to FIG. 2, the different operative states of the friction elements as well as an engine brake according to shift range and shift speeds within the ranges (where applicable) are shown. The shifting operation of the power train will be described with reference to FIG. 1 and the chart of FIG. 2.
In a first speed, the first clutch C1 and the first and second one-way clutches F1 and F2 are operated. Accordingly, the first sun gear 12 acts as an input element, and the first ring gear 14 and the second planet carrier 18 act as reaction elements. Shifting into a second speed from the first speed is realized by operation of the second brake B2. That is, through the engagement of the second brake B2, input is realized through the first sun gear 12, and the second sun gear 16 acts as a reaction element such that shifting into the second speed is realized.
Shifting into the third speed from the second speed is realized by operation of the second clutch C2 and disengagement of the second brake B2. As a result, the first and second single pinion planetary gear sets 4 and 6 are linked such that output that is identical to the input results. Shifting into the fourth speed (i.e., overdrive) from the third speed is realized by the operation of the second brake B2 such that the second sun gear 12 acts as a reaction element.
To effect shifting into the reverse R range, the third clutch C3 and the first brake B1 are controlled to engaged states such that input is realized through the second sun gear 16 and the second planet carrier 18 acts as a reaction element.
In sum and to describe operational states of the friction elements for ranges not yet mentioned, shifting is realized as follows: the first clutch C1 operates in the first, second and third speeds; the second clutch C2 operates in the third and fourth speeds; the third clutch C3 operates in the reverse R range; the fourth clutch C4 operates in the park P, reverse R, neutral N and low L ranges, and as needed in the first, second and third speeds; the first brake B1 operates in the park P, reverse R, neutral N and low L ranges; and the second brake B2 operates in the second and fourth speeds.
With reference to FIG. 6, in a hydraulic control system for controlling the power train above, a D range pressure from a manual valve 200 is supplied to the first clutch C1 and to first, second, and third pressure control valves 202, 204, and 206. Also, an L range pressure is supplied to the first pressure control valve 202, and an R range pressure is supplied to the third clutch C3 and the first brake B1.
In addition, the D range pressure supplied to the first pressure control valve 202 is selectively supplied to an operational side of the second brake B2, according to control by the first solenoid valve 208, and the L range pressure is supplied to the first brake B1 in the low L range. The first brake B1 is connected to the first pressure control valve 202 and an R range port of the manual valve 200 via a shuttle valve 210 such that hydraulic pressure is supplied to the first brake B1 no matter which direction hydraulic pressure is supplied from. The D range pressure supplied to the second pressure control valve 204 is supplied to the second clutch C2 and the third pressure control valve 206 according to control by a second solenoid valve 212. Also, the D range pressure supplied to the third pressure control valve 206 is selectively supplied to the fourth clutch C4 according to control by a third solenoid valve 214. In such an instance where the D range pressure is supplied to the fourth clutch C4, the third pressure control valve 206 supplies hydraulic pressure from the second pressure control valve 204 to a non-operational side of the second brake B2.
However, in the conventional hydraulic control system as described above, since the system acts simply to control line pressure and the solenoid valves merely operate as switch valves to control timing, precise shift control is not possible. In particular, in the shift between the second and third speeds, since there is used a method of control in which the second brake B2 is disengaged when the second clutch C2 is engaged, precise control during shifting is not possible. Also, with the operation of the first brake B1 and the fourth clutch C4, which enable operation of the engine brake, since a method is used in which line pressure is directly supplied, significant shift shock is generated.
Further, during manual shifting from the low 2 range to the low L range, occurring simultaneously with the exhaust of operational side pressure of the second brake B2, is the supply of line pressure to the first brake B1 resulting in the generation of substantial shift shock. Shifting into the reverse R range from the drive D range when travelling at a high speed results in shifting being forcedly performed by line pressure, as well as possible damage to friction material.
In addition, if manual control into the low L range is performed when driving in the third or fourth speeds, engine rpm is excessively increased by the disengagement of the second clutch. Accordingly, the engine control unit abruptly performs engine fuel cut-off to protect the engine. However, shifting into neutral occurs during this control such that normal operation of the vehicle is not possible.
The present invention provides a hydraulic control system for an automatic transmission, in which two one-way clutches are used in a four-speed automatic transmission and the advantages of the one-way clutches are able to be optimally used during 1⇄2, 3⇄4, and 4⇄2 shifting.
An exemplary automatic transmission power train useful with the present invention includes first, second, third, and fourth clutches operating, respectively, when in first, second, and third speeds, when in third and fourth speeds, when in a reverse R range, and when an engine brake is required. The power train also includes a first brake, operating when the engine brake in the first speed is required or in the reverse R range, and a second brake operating in the second and fourth speeds.
Thus, according to a preferred embodiment of the invention, a hydraulic control system for a power train of an automatic transmission comprises at least first and second control valves communicating with a hydraulic pressure source, at least first and second solenoid valves communicating with the first and second control valves, respectively, to supply a control pressure thereto, at least first and second switch valves communicating with the first and second control valves, and a common third solenoid valve communicating with both the first and second switch valves to supply a control pressure thereto. In this embodiment, a first hydraulic pressure is selectively supplied to two clutches of the power train via the first control valve and under control of the first switch valve. A second hydraulic pressure is selectively supplied to another clutch and a first brake of the power train via the second control valve and under control of the second switch valve. Preferably, a manual valve is provided through which hydraulic pressure is supplied from the pressure source for selecting desired gear ranges.
In a further preferred embodiment, third and fourth control valves communicate with the hydraulic pressure source and a fourth solenoid valve communicates with both the third and fourth control valves to supply a control pressure thereto. The first hydraulic pressure is provided to another brake under control of the third control valve and third hydraulic pressure is provided to another clutch under control of the fourth control valve.
A hydraulic control system according to another preferred embodiment of the invention comprises a manual valve and a plurality of switch and control valves cooperating with solenoid valves. The manual valve includes an R range port for exhausting hydraulic pressure in the reverse R range, an N range port for exhausting hydraulic pressure in all shift ranges except the reverse R range, a D range port for exhausting hydraulic pressure in all forward driving ranges, and an L range port for exhausting hydraulic pressure in low range. A first switch valve selectively supplies a first pressure, which is controlled by a first solenoid valve, to the first clutch and the fourth clutch. A second switch valve selectively supplies a second pressure, which is controlled by a second solenoid valve, to the second clutch and the first brake. A third-clutch control valve controls hydraulic pressure received from the R range port of the manual valve and supplies the hydraulic pressure to the third clutch. A second-brake control valve controls hydraulic pressure supplied from the D range port of the manual valve and supplies the hydraulic pressure to the second brake. A third solenoid valve simultaneously controls the third-clutch control valve and the second-brake control valve.
According to a preferred embodiment of the present invention, the first switch valve supplies D range pressure to the first clutch when the first pressure is supplied to the fourth clutch, and supplies line pressure to the fourth clutch when the first pressure is supplied to the first clutch.
According to another preferred embodiment of the present invention, the first switch valve is controlled by a fourth solenoid pressure operating on one side and the D range pressure operating on an opposite side.
According to yet another preferred embodiment of the present invention, the first switch valve supplies the first pressure to the fourth clutch in the case where the D range pressure is greater than the fourth solenoid pressure.
According to still yet another preferred embodiment of the present invention, the second switch valve exhausts hydraulic pressure supplied to the second clutch when the second pressure is supplied to the first brake, and exhausts hydraulic pressure supplied to the first brake when the second pressure is supplied to the second clutch.
According to still yet another preferred embodiment of the present invention, the second switch valve is controlled by D range pressure operating on one side and by L range pressure and solenoid pressure operating on an opposite side, and the conversion of port communication is able to be accomplished by operation of both the L range pressure and the solenoid pressure in the case where the D range pressure is operating on the second switch valve.
According to still yet another preferred embodiment of the present invention, the solenoid pressure is pressure of a fourth solenoid valve.
According to still yet another preferred embodiment of the present invention, the second switch valve supplies the second pressure to the second clutch when the D range pressure is greater than a sum of the L range pressure and the solenoid pressure.
According to still yet another preferred embodiment of the present invention, the hydraulic control system further comprises a fail-safe valve mounted on a line extending between the second brake and the second-brake control valve, the fail-safe valve for blocking off a line between the second brake and the second-brake control valve when hydraulic pressure is supplied to the first brake or when hydraulic pressure is supplied simultaneously to the second clutch and the fourth clutch.
According to still yet another preferred embodiment of the present invention, the fail-safe valve is controlled by D range pressure supplied to one side, and by first brake supply pressure, second clutch supply pressure, and fourth clutch supply pressure supplied to an opposite side, the fail-safe valve exhausting hydraulic pressure supplied to the second brake in the case where the first brake supply pressure or a sum of the second and fourth clutch supply pressures is greater than the D range pressure, and, if this condition is not satisfied, communicating the second-brake control valve with the line of the second brake.
According to still yet another preferred embodiment of the present invention, the second pressure is hydraulic pressure exhausted from a second pressure control valve, which receives hydraulic pressure from the N range port of the manual valve and is controlled by the second solenoid valve.